This invention relates to an ignition plug for an internal combustion engine.
In order to improve emissions in petrol internal combustion engines, the engine may be operated with exhaust gas recirculation (EGR) or lean air-fuel mixtures. However, combustion stability may become unacceptable under these conditions, when using known spark plugs and ignition systems. One reason for this is the small volume of gas that is conventionally being ignited across the spark gap (typical 0.8 mm) of a known spark plug.
It has been shown that the combustion stability may be improved by igniting a larger volume of gas by using larger gap corona ignition systems. However, such systems require higher voltages, which often are problematic. For example, the higher voltages may lead to breakthrough in the ceramic body of the plug, back arcing in the plug, etc. Some known corona systems comprise sharp electrodes extending into the combustion chamber for igniting a larger volume of gas. However, these electrodes may become too hot under certain conditions, which may result in combustion at inappropriate times. Furthermore, with the high voltages (typically about 100 kV) and associated heat, wear of the electrodes become a problem.
Still furthermore, sparks may also occur in corona systems, causing damage to the sharp electrodes. In conventional corona systems, sparking has to be prevented as far as possible to minimise electrode wear and should sparking occur, it has to be controlled to achieve good combustion (see for example U.S. Pat. Nos. 8,578,902 and 8,726,871). As the electrode wears, it becomes blunt, and higher voltages are required to achieve a corona.